Abstract. The aim of this paper is to study the impacts of overshooting convection at
a local scale on the water distribution in the tropical UTLS. Overshooting
convection is assumed to be one of the processes controlling the entry of
water vapour mixing ratio in the stratosphere by injecting ice crystals
above the tropopause which later sublimate and hydrate the lower
stratosphere. For this purpose, we quantify the individual impact of two
cases of overshooting convection in Africa observed during SCOUT-AMMA: the
case of 4 August 2006 over Southern Chad which is likely to have
influenced the water vapour measurements by micro-SDLA and FLASH-B from
Niamey on 5 August, and the case of a mesoscale convective system
over Aïr on 5 August 2006. We make use of high resolution (down
to 1 km horizontally) nested grid simulations with the three-dimensional
regional atmospheric model BRAMS (Brazilian Regional Atmospheric Modelling
System). In both cases, BRAMS succeeds in simulating the main features of
the convective activity, as well as overshooting convection, though the
exact position and time of the overshoots indicated by MSG brightness
temperature difference is not fully reproduced (typically 1°
displacement in latitude compared with the overshoots indicated by
brightness temperature difference from satellite observations for both
cases, and several hours shift for the Aïr case on 5 August
2006). Total water budgets associated with these two events show a
significant injection of ice particles above the tropopause with maximum
values of about 3.7 ton s−1 for the Chad case (4 August) and 1.4 ton s−1
for the Aïr case (5 August), and a total upward
cross tropopause transport of about 3300 ton h−1 for the Chad case and
2400 ton h−1 for the Aïr case in the third domain of simulation.
The order of magnitude of these modelled fluxes is lower but comparable with
similar studies in other tropical areas based on models. These two
estimations exhibit significant differences and highlight variability among
the cases of the impact of overshooting convection in hydrating the lower
stratosphere. We show that the regional enhancement of water above the
tropopause is between 0.21 to 0.67 ppmv between 380 and 400 K, generally in
the range of other model estimations. The amount of water which remains in
the stratosphere after the overshoot is estimated for both cases. A range of
330 to 507 tons is found for the Chad case and an upper limit of 200 tons is
found for the Aïr case. Finally we emphasize that the hydrated area in
the LS by overshooting convection can be advected relatively far away from
the overshoot initial location, with locally mixing ratios of more than 3 ppmv
higher than the background level, which is compatible with the balloon
borne measurements performed above Niamey in the same air mass, 30 h
after the overshoot.